The present invention relates to a CCD device, and in particular to a CCD imager which provides gain in a CCD channel.
In a typical CCD imager, signal charge representative of incident radiation is accumulated in an array of pixels in an image area. Following an integration period, signal charge is transferred to a store section and then to an output register by applying appropriate clocking or drive pulses to control electrodes. The signal charge is then read out from the output register and applied to a charge detection circuit to produce a voltage which is representative of the amount of signal charge. The sensitivity of such a device is limited by the noise of the charge to voltage conversion process and that introduced by the subsequent video chain electronics.
An electron multiplying CCD overcomes this limitation and is disclosed in our earlier published UK patent application GB-A-2,371,403, as shown in FIG. 1. A CCD imager 1 comprises an image area 2, a store section 3 and an output or read-out register 4, each of these components being found in a conventional CCD imager. The output register 4 is extended serially to give a multiplication register 5, the output of which is connected to a charge detection circuit 6.
During operation of the device, incident radiation is converted at the image area 2 into signal charge which is representative of the intensity of the radiation impinging on the array of pixels making up the image array. Following the image acquisition period, drive pulses are applied to control electrodes 7 to transfer the charge accumulated at the pixels of the image area 2 to the store section 3. Simultaneously with this, drive signals are also applied to control electrodes 8 at the store section 3 to cause charge to be transferred from row to row as indicated by the arrow, the last row of charge held in elements in row 3 being transferred in parallel to the output register 4.
When a row of signal charge has been transferred into the output register 4, appropriate drive pulses are applied to the electrodes 9 to sequentially transfer the charge from the elements of the output register to those of the multiplication register 5. In this embodiment, the multiplication register is of similar architecture to the output register in so far as doping is concerned with the addition of an electrode for multiplication. To achieve multiplication of charge in each of the elements of the multiplication register 5, sufficiently high amplitude drive pulses are applied to control electrodes 10 to both transfer signal charge from one element to the next adjacent element in the direction shown by the arrow and also to increase the level of signal charge due to impact ionisation by an amount determined bv the amplitude of the drive pulses. Thus, as each packet of charge is transferred from one element to the next through the multiplication register, the signal charge increases. The charge detected at circuit 6 is thus a multiplied version of the signal charge collected in the output register 4. At each stage of the multiplication register, the signal charge is increased. Each signal charge packet stored in the output register 4 undergoes an identical multiplication process as each travels through all the elements of the multiplication register 5.
The output of the charge detection circuit 6 is also applied to an automatic gain control circuit 11 which adjusts the voltages applied to the multiplication register 5 to control the gain. In other embodiments, this feedback arrangement is omitted. Gain may then be controlled manually if desired.
We have appreciated that improvements can be made to the gain performance of CCD devices which provide gain in a CCD channel. In particular, we have appreciated that the gain performance of such a CCD device can deteriorate with time of use.